Drop-on-demand thermal ink jet printers use thermal energy to produce a vapor bubble in an ink-filled chamber to expel a droplet. A thermal energy generator or heating element, usually a resistor, is located in the chamber on a heater chip near a discharge orifice. A plurality of chambers, each provided with a single heating element, are provided in the printer's printhead. The printhead typically comprises the heater chip and a nozzle plate having a plurality of the discharge orifices formed therein. The printhead forms part of an ink jet print cartridge which also comprises an ink-filled container.
The resistors are individually addressed with an energy pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. A flexible circuit is used to provide a path for energy pulses to travel from a printer energy supply circuit to the printhead. The flexible circuit includes a substrate portion and a plurality of traces located on the substrate portion. The traces have end sections which extend out from the substrate portion. The extending sections are coupled to bond pads on the printhead. Typically, there is a first row of coupled bond pads and trace sections and an opposing, second row of coupled bond pads and trace sections.
It is known in the art to form a barrier layer over each row of coupled bond pads and extending trace sections. One known process for forming such a barrier layer involves dispensing an encapsulant material onto the coupled bond pads and trace sections using a discharge needle. The final height of the barrier layer relative to the nozzle plate typically is undesirably high. As a result, a paper substrate, which receives the ejected ink droplets, is spaced an increased distance from the printhead orifice plate. Consequently, misdirected ink droplets reach the paper substrate at locations which are spaced a greater distance from their intended contact points than if the paper substrate were located closer to the printhead orifice plate. The excessive height of the barrier layer is further problematic as it makes it more difficult to apply a length of sealing tape to the printhead so as to seal the printhead orifices from ink leakage until the print cartridge is installed for use in a printer. Another potential problem associated with dispensing an encapsulant material with a discharge needle relates to improper location. Dispensing encapsulant in the wrong locations can result in unacceptable product because the encapsulant fails to provide the necessary coverage for the electrical components on the print cartridge or covers areas intended to be free of encapsulant such as for ink ejection or capping purposes.
The ability to properly place an adhesive having the proper dimensions is also an issue with respect to adhesives used for die attach, potting, etc. As the components used to manufacture the inkjet printheads undergo miniaturization, the assembly process faces many challenges in placing components with high precision and holding them under tight tolerances. Stencil printing is used in the electronics industry for solder paste deposition. The precision and holding accuracy of the final structure depends not only on the handling, placement capability of the equipment, and substrate properties but also on the material characteristics of the flux, adhesives or solder paste being used. For most materials, the flow and deformation characteristics both before and during cure are critical. Placement accuracy and final location of the chip become more critical as the die dimensions are miniaturized. Potting and encapsulation of TAB beams and interconnects are also affected when the adhesives have to be dispensed in smaller spaces and need to be held to tighter dimensional tolerances.
Flow properties of adhesives primarily depend on the chemical and physical structure and hence on their composition. Depending on the application, the requirements for flow and deformation of adhesives may be different. For example, non-contact needle dispense systems require the adhesive viscosity to be low enough that flow through the needle is sufficient. However, depending on the shapes and sizes to be filled, or placed, the adhesive may need to have specific properties with respect to flow-out, recovery and elasticity. For stencil printing, it is critical that the adhesive shear thins while filling the apertures. The adhesive must then recover and retain the shape, geometry and dimensions after printing. In addition to the above properties, if the material is a crosslinkable, thermosetting material, the cure parameters can affect the flow and deformation of the adhesive thereby dictating the final dimensions and tolerance. For thermally curable adhesives, the flow and deformation during curing under heat or heat and pressure affects the resulting dimensions. Therefore, in order to maintain tolerances for these placement and dimensions, it is important to have an adhesive with the appropriate rheology for the application before and during curing of an adhesive.
In an inkjet printhead assembly, it is critical to maintain the location of the chip after curing of the die attach adhesive. After the adhesive is dispensed or stencil or screen printed, the chip is aligned and placed on top of the adhesive before the adhesive is cured. During the cure, the chip location can shift depending on the rheology of the die attach adhesive under temperature and load. Another application wherein it is important to maintain adhesive location and dimensions is the encapsulation of TAB beams with a thermally curable adhesive. During the curing cycle of the encapsulant adhesive, the shape and dimensions of the adhesive bead may change. Proper location and dimensions are also important for applying an underfill material where it is essential that the material spreads and fills the chip cavity and may require flow characteristics very different from the die attach or the encapsulant materials.
Commonly assigned U.S. Pat. No. 6,439,698 describes a method of stencil printing an encapsulant material over electrical connections and other areas on an inkjet printhead. The encapsulant is applied to the electrical connections preferably in the form of a bead.
Commonly assigned U.S. patent application Ser. No. 10/679,070 describes a method of stencil printing an encapsulant material over electrical connections and other areas on an inkjet printhead.